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Research on the Adaptability of Helicoidal-Ramp Type Drop Shafts rampe hélicoïdale

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Research on the Adaptability of Helicoidal-Ramp Type Drop Shafts rampe hélicoïdale
NOVATECH 2010
Research on the Adaptability of Helicoidal-Ramp
Type Drop Shafts
Recherche sur l'adaptabilité des puits de chute de type
rampe hélicoïdale
Yuichiro NOBUSAWA1, Hironobu NISHIMURA1, Shizuo
YOSHIKAWA1, Masaharu KATOU 2, Hitoshi Ikenaga (Dr.Eng..)3
and Isao Nihei 4
1
JIWET: Japan Institute of Wastewater Engineering Technology, 3-1 Suido-cho,
Shinjuku-ku, Tokyo 162-0811, Japan ([email protected])
2
Sekisui Chemica Co.,Ltd. : 75 Nogiri, Ritto-shi, Shiga 520-3081, Japan
([email protected])
3
Nippon Koei Co.,Ltd. : 2304,Inarihara, Tsukuba-shi, Ibaraki 300-1259 Japan
([email protected])
4
CTI Engineering Co.,Ltd. 1047-27 Onigakubo, Tsukuba-shi, Ibaraki 300-1259
Japan ([email protected])
RÉSUMÉ
Un puits de chute de type rampe hélicoïdale installé sur des regards de visite de systèmes
d’assainissement (appelés ci-après "DRS") est un ouvrage de chute à haute charge qui dissipe
l’énergie hydraulique pour que l’eau atteigne le fond sans force excessive. Toutefois, l’augmentation
des profondeurs et des débits, la diversification des méthodes de transfert des eaux vers les DRS, et
les contre-mesures d’entraînement d’air ont fait apparaître de nouveaux problèmes. Dans cette étude,
la direction des flux entrants et les contre-mesures d’entraînement d’air ont été examinées et vérifiées
par des essais sur modèles hydrauliques.
La partie supérieure d’entrée d’eau d’un DRS est généralement un raccordement cylindrique à une
conduite, mais une autre méthode de raccordement aux réservoirs d’eau est de plus en plus utilisée :
au niveau des conduites aval, les méthodes actuelles posent le problème des impacts des remous qui
endommagent les installations et génèrent du bruit et des vibrations. Pour remplacer cette approche,
un raccordement par mur déflecteur a été examiné. Les entraînements d’air associés au débit d’entrée
d’eau sur des regards de visite de grande hauteur peuvent causer des problèmes tels que le
dysfonctionnement des conduites, des remous ou des dommages aux regards de visite. Les essais
ont permis de vérifier l'utilité de systèmes d'aération et d'acquérir de nouvelles données sur la
longueur requise des conduites de prise d’air en fonction de la distance de flottaison de l’air dissout, et
l’efficacité des conduites d'assainissement installées au droit des conduites de prise d’air.
ABSTRACT
A helicoidal-ramp type drop shaft installed at high head drop connection manholes on sewage
systems (below referred to as “DRS”) is a high head drop work which efficiently absorbs energy to
allow water to fall gently to the bottom. However, a variety of new problems have appeared as the
depths and flow discharge have increased, methods causing water to flow into DRS diversified, and air
entraining countermeasures have been taken. In this research, the direction of the inflow and air
entraining countermeasure was examined and verified by hydraulic model testing.
The top inflow part of a DRS is basically a cylindrical pipe connection, but another water tank
connection method is coming into wider use. Concerning tank connection, current methods have
problems which are backwater impacts on the upstream pipe, damaging facilities and producing
vibration and noise. As an inflow shape to replace this approach, a training wall connection method
was examined.
Air-entraining according to water from a high-head manhole can cause problems such as a decrease
in the pipe’s functioning, gush of the sewage or damage of the manhole. The testing verified an air
exhaust system’s functions and obtained new knowledge concerning the required length of the air
intake pipes according to the dissolved air flotation distance, and the effectiveness of drainage pipes
installed at bends in the air intake pipes.
KEYWORDS
High head drop work, drop shaft, increasing depth, training wall connection method, entrained air
countermeasure
1
SESSION 2.3
1
PURPOSE
In recent years, the depth of underground drainage sewer pipes has been steadily increasing. The
Helicoidal Ramp Type Drop Shaft (DRS) method is increasingly used in high head drop applications.
Meanwhile, the uses of DRS have also changed, with a variety of new challenges posed by greater
depths and higher flow rates, increasingly diverse methods of channelling water flows into DRS,
actualization of problems in construction and maintenance, and air entraining countermeasures.
This study seeks to investigate these challenges and verify them through hydraulic model testing.
2
ABSTRACT OF DRS
DRS is composed of an Entrance Section, Upper Helicoidal Ramp, Middle Hollow Section, Lower
Helicoidal Ramp, and Exit Section (Figure 1). It absorbs energy efficiently and allows water to fall
gently to the bottom by creating and sustaining an artificial spiral flow in the Helicoidal Ramp. DRS
prevents scouring of manholes, reduces the quantity of entrained air, mitigates noise and vibration,
allows a smaller manhole area, and enables work to be completed more quickly.
Upper Helicoidal-Ramp
・Guide the flow spirally along
the dropshaft wall.
Helicoidal Multi-level Thrust type Vortex
-ramp
freefall type
flow type
dorop shaft high head high head high head
Lower Helicoidal-Ramp
・Reduce the dropping energy of flow
by the lower helicoidal-ramp.
Figure 1 Basic Structure and Features of DRS
3
3.1
Upper
helicoidal
ramp
Outline diagram
[Special features]
■Reduces quantity of entrained air
■Mitigates noise and vibration
■Prevents scouring of the manhole
■Reduces surface area of manhole
■Shortens work period, etc.
Angled
pipe
Air
extractor
Middle
hollow
section
Separating
wall
Lower
helicoidal
ramp
Angled pipe
Drop shaft
Stilling pool
Figure 2 DRS and other high head drop schema
RESEARCH CONTENT
Flow discharge measures
The first stage of the study involved a questionnaire survey with municipal government authorities in
Japan on the diameter of DRS used (Figure 3). It can be seen that there were no results in excess of
2,800 mm in the period up to 2002.
Therefore, a diameter of 2,800 mm or less has provided coverage thus far. This figure has been
3
increased to 3,000 mm, resulting in a flow discharge increase of approximately 3 m /s
Ratio of the diameter of DRS (%)
0
50
60
70
80
90
100
H8年
1996
H10年
1998
H12年
2000
2002
H14年
H16年
2004
H18年
2006
(year)
<1000mm <2000mm <2800mm 2800mm≦
Figure 3 Diameter of DRS used (in Japan)
2
NOVATECH 2010
3.2
Correspondence to head drop
Figure 4 shows the survey findings on head drop.
Results for head drop in excess of 40 m are not shown here.
The Middle Helicoidal Ramp has been developed in anticipation
of deeper installations in the future. As shown in Figure 5 ,
it is designed to replicate the helical flow
in the Middle Hollow Section. It has been trialled in the past
and the specifications have been validated.
For a head drop 28 times (28D) the DRS diameter
the Middle Helicoidal Ramp was trialled. Therefore,
it should be possible to provide up to 84 m
as the maximum application caliber of 3,000 mm.
Core tube
Guide plate
Inlet
Inflow
section
Upper
helicoidal
ramp
Upper
helicoidal
ramp
Upper
middle
ramp
Main body
Middle
helicoidal
ramp
20~40m 8.1%
Lower
middle
ramp
10~20
25.7%
Lower
helicoidal
ramp
2.4~10m
66.2%
Outlet
Exit
section
Figure 4 Head drop by usage
Lower
helicoidal
ramp
Figure 5 Middle Helicoidal Ramp type DRS
On the other hand, past coverage is assumed to be about 45 m or less based on head drop 16.5D
and a maximum application caliber of 2,800 mm to be able to apply the internal extractor type DPS.
Since a separate review was not conducted, it was assumed to be about 45 m or less for the purpose
of this study. This is because there are no instances of construction in excess of 45 m to date, and it is
not sufficient to verify the strength of the material and the volume of air.
3.3
3.3.1
Diversification of inflow methods(hydraulic model testing of tank
connection methods)
Background and purpose
The top inflow part of DRS is basically a cylindrical pipe connection. The trend is increasingly towards
water tank connections where the tank-shaped wash bulkhead is installed such that water flows in
from all directions. This approach offers superior workability in relation to connection and less
restriction on the diameter of manhole (see Figure 6). However, there is little consistency of shape due
to the lack of design specifications. This study therefore seeks to verify the water tank connection
method through experimentation in order to determine the optimum shape.
Drop
shaft
Guide
plate
Entering
Section
Drop shaft
Guide
plate
Entering Section
Figure 6 Top connection to DRS
3
SESSION 2.3
3.3.2 Problem
The water tank connection method allows inflow from all directions. This differs from the direct tube
connection method with single direction insertion as shown in Figure 7.
Flow decreases when changing from the open channel under pressure, and the water level of the
water tank rises. Flow increases when the water level rises and a large volume flows at once; as the
water level decreases, the flow decreases in proportion. The water level changes through this
repetition as depicted by the arrows in Figure 8 (vertical direction), and this can potentially cause a
pulsing effect.The mixed air also contributes to this complex behavior.
図-7 全集方向流入
Figure 7 Intake flow from all direction
3.3.3
図-8 水位変動イメージ
Figure 8 Functuation in water level
Experiment case
3.3.3.1 Inflow shape
It is thought that the problem could be addressed by installing a training wall as shown in Figure 9,
to make the flow of the inflow part one direction and obtain the round tube connection method with the
same water level and flow quantity.
To this end, the experiment was conducted with the case of the water tank connection method and the
second case (training wall connection method) with a training wall installed.
Training wall
Training wall
[Left eccentricity]
Figure 9 Positioning of training wall
Training wall
[Right eccentricity]
Figure 10 Positioning of training wall for eccentricity case
3.3.3.2 Direction of inflow
The results show a further case with DRS at an off-center position as shown in Figure 10. The
experiment with the training wall installed verified the case with eccentricity (both left and right).
3.3.3.3 Flow discharge condition
Flow discharge was assumed to increase (1.2 Qd) by approximately 20% above the design flow
discharge (1.0 Qd) to a steady shape in addition to the round pipe connection concept.
3.4
Experimental equipment
Figure 11 shows the experimental equipment, while Figure 12 illustrates the experimental design.
The water tank parameters were selected on the basis of the highest occurrence rate among 76
results (no eccentricity was 84% in 76 results). Eccentricity was assumed to be a condition of moving
the center of DRS to the right and left 0.2Ds relative to the intake pipe center axis. (Eccentricity of
<=0.2Ds was 95% in 76 results.).
4
NOVATECH 2010
<With eccentricity>
<偏心させる場合>
Dry area
水槽
tank design>
<水槽部概要>
Holding tank <Holding
Main storage tank
Connected to
holding
tank
連結水槽
(wooden)
(木製)
Holding tank
Main storage
tank
水タンク
Φ160
DRS
(Upper
only shown)
(上部のみ再現)
<Equipment
specifications>
<設備諸元>
図-12
Figure 11 Experimental setup
3.5
3.5.1
実験設備概要
Figure 12 Experimental design
Results of experiment
Round pipe connection method (Standardized form、Reference)
As Figure 13 shows, in the wide range exceeding 1.0 Qd – 1.2 Qd, a steady stream regime can be
attained in the round tube connection method that is the state of the standardized form.
Water depth ratio (H/Din)
1.2Q
ΔH/Din
2.0
1.0Q
max
min
1.0Qd curve
1.20Qd curve
ΔH/Din
1.5
2.0
1.5
1.0
1.0
0.5
0.5
0.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Flow discharge ratio (Q/Qd)
Figure 13 DRS stage-discharge characteristics-cylindrical pipe, stand profile
3.5.2
Water tank connection method
・Circle: As Figure 14 shows, flows greater than 1.0 Qd cause fluctuations of the water surface at the
length cycle in the water tank. (The backwater effect on the upstream pipe, facilities damage, vibration
and noise was caused by the pressure fluctuation.)
5
SESSION 2.3
ΔH/Din
Water depth ratio (H/Din)
2.0
1.2Q
2.0
max
min
1.0Qd curve
1.20Qd curve
Unstable area
ΔH/Din
1.5
1.0
1.5
1.0
0.5
0.5
0.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Flow discharge ratio (Q/Qd)
1.4
1.6
Figure 14 Holding tank stage-discharge characteristics-holding tank, cylindrical profile
・ Rectangle: As Figure 15 shows, flows greater than1.0 Qd cause fluctuations of the water surface at
the length cycle in the water tank. However, the phenomenon differs considerably from circle case
from the water level flow discharge characteristic in the water tank. Standardization of the water tank
connection method is difficult because the inflow part stream regime is highly dependent on the water
Δ H/Din
tank shape.
1.2Q
Water depth ratio (H/Din)
2.0
2.0
max
min
1.0Qd curve
1.20Qd curve
Unstable area
ΔH/Din
1.5
1.5
1.0
1.0
0.5
0.5
0.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Flow discharge ratio (Q/Qd)
1.4
1.6
Figure 15 Holding tank stage-discharge characteristics-holding tank, rectangular profile
3.5.3 Training wall connection method
・Training wall method (no eccentricity): Steady flows up to 1.2 Qd were obtained through installation
of a training wall as shown in the following photograph (upper row) (see Figure 16).
ΔH/Din
2.0
1.2Q
Water depth ratio (H/Din)
2.0
max
min
1.0Qd curve
1.20Qd curve
Unstable area
Δ H/Din
1.5
1.5
1.0
1.0
0.5
0.5
0.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Flow discharge ratio (Q/Qd)
1.4
1.6
Figure 16 Holding tank stage-discharge characteristics-training wall, no eccentricity
・Training wall method (left eccentricity): In the left eccentricity case, unstable regions were prevented
at flows of up to 1.2 Qd by installing a training wall as shown in the following photograph (see Figure
17).
6
NOVATECH 2010
Δ H/Din
2.0
2.0
1.0Ds
0.2Ds
Water depth ratio (H/Din)
max
min
1.0Qd curve
1.20Qd curve
1.5
1.5
Unstable area
ΔH/Din
1.0
1.0
0.5
0.5
0.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Flow discharge ratio (Q/Qd)
1.4
1.6
Figure 17 Holding tank stage-discharge characteristics-training wall, left eccentricity
・ Training wall method (right eccentricity): In the right eccentricity case, unstable regions were
prevented at flows of up to 1.2 Qd by installing a training wall as shown in the following photograph
Δ H/Din
(see Figure 18).
Water depth ratio (H/Din)
2.0
2.0
max
min
1.0Qd curve
1.20Qd curve
Unstable area
ΔH/Din
1.5
1.5
1.0
1.0
0.5
0.5
0.2Ds
0.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Flow discharge ratio (Q/Qd)
Figure 18 Holding tank stage-discharge characteristics-training wall, right eccentricity
3.6
Summary
In situations where the round pipe connection method cannot be used, the training wall connection
method provides a suitable alternative because the water tank connection method is unstable in
hydraulics.
4
AIR-ENTRAINING COUNTERMEASURES
(HYDRAULIC MODEL TESTING OF AN AIR EXHAUST SYSTEM CONSISTING
OF AIR INTAKE PIPES)
4.1
Background and purpose
The use of storage pipes and long, large inverted siphon pipes that are under pressure during
rainfall may be the cause problems such as decreased pipe efficiency, sewage gushing and damage
to manholes (scattering of manhole lids, and manhole structures and pavements coming to the
surface) associated with water from high head manhole impacting on structure and safety.
It is believed that the problem could be resolved by reducing the volume of air taken from the high
head manhole to the pipe.
An example of equipment used for air-entraining in a high head is the air exhaust system developed
by JIWET (Patent application, Special opening2003.253739), which includes air intake pipes. The
system was verified in a hydraulic model experiment that confirmed the high exhaust performance.
Meanwhile, the current air intake pipe design has been verified by hydraulic model experiments at
individual facilities. As a result, there is no standard design method corresponding to the size of the
facility and air intake pipe.
This experiment uses hydraulic model testing to verify the parameters of the air intake pipe and the
impact of collection air efficiency on hydraulics, and generates hydraulics data on the functionality of
air intake pipes.
7
SESSION 2.3
4.2
Air intake pipe
An air exhaust system of air intake pipes, consisting of porous air intake pipes (approximately 5 mm
in diameter), an exhaust pipe and the curtain, is installed at the downstream pipe connected to the
high head manhole, where it captures entrained air and expels it from the system (see Figure 19).
⑤Entrained air is expelled from
the system via exhaust pipe
④Movement of confined air
(confined air pressure>manhole pressure
(atmospheric pressure))
①Air mixing and entrainment
at high head facility
③Confined air rises to top of pipe
and enters collecting pipe
②Bubbles surface
within connecting pipe
Figure 19 Exhaust system with air intake pipe
4.3
Experimental methodology
The experimental equipment in connecting pipe is shown in Figure 20. The experimental conditions
are outlined in Figure 21 and Table 1.
High head manhole
curtain
Water supply1
(drop shaft)
Water supply2
Collector
Entrained air sent downstream
Connecting pipe length 6D、10D
Collector pipe size 5mm
Collector pipe diameter 1/10D
Flow velocity 1m/s, 2m/s, 3m/s
Connecting pipe diameter D≒2.5m
Air mixing at 5%,10% of flow discharge
air intake pipe
Figure 20 Outline of experiment
Curtain 1 type
Model scale: 1/8
Figure 21 Experimental conditions(Showing)
equipment in connecting pipe
Table 1 Experimental conditions
Hydraulic
conditions
Collector pipe
conditions
Measured
items
Water depth in main pipe 100%:pipe full
Flow velocity
1.0(0.35),2.0(0.71),3.0(1.06)m/s
Air mixing ratio
5.0%, 10.0% of flow discharge
Internal diameter φ250mm(Φ31mm):1/10 of connecting pipe diameter
Pore diameter
φ5mm
Fixed
Pole distribution open area ratio2%,pores on 2/3 section of downstream side
conditions
Curtain height 500mm: 1/5 of connecting pipe diameter
Extended(collector pipe length) → TypeⅠ[6D,D:connecting pipe diameter]
TypeⅡ[10D,D:connecting pipe diameter]
Comparison
Modified(drain)
→ Type A[drain pipe]
conditions
Type B[external discharge]
Water level,observation of flow,air mixture quantity,collected air quantity
※In parentheses, it is a model value.
8
NOVATECH 2010
4.4
Experimental outcomes
高落差人孔
High
head manhole
Bubble surfacing
気泡浮上距離 distance
Main pipe
幹線
F low
Flow
接続管
Connecting
pipe
Bubble surfacing distance in model (m)
模型の気泡浮上距離(m)
4.4.1 Bubble surfacing distance in connecting pipe
The experiment illustrated the bubble surfacing distance in the connecting pipe in each pass flow
discharge condition. As Figure 22 shows, there is a linear relationship between flow velocity in the
connecting pipe and the bubble surfacing distance in the model.
3.0
y = 3.1516x - 0.5365
2.5
2.0
1.5
1.0
0.5
*Connecting
pipe diameter in model D=0.3m
※接続管の模型管径D=0.3m
0.0
0.0
0.2
0.4
0.6
0.8
1.0
接続管の模型流速(m/s)
Flow velocity
in connecting pipe in model (m/s)
1.2
Figure 22 Flow velocity in connecting pipe versus bubble surfacing distance (Model value)
4.4.2 Efficiency of air collection at installed air intake pipe
Table 1 shows the air content in each pass flow discharge condition of two types for different lengths
of air intake pipe.
Figure 23 shows the outcomes from experiment types I and II. In experiments performed with flow
velocity of 1.0 m/s (flow velocity 0.35 m/s in the model) and 2.0 m/s (0.71 m/s in the model), all the air
taken from the manhole into the connecting pipe surfaces on the upstream side of the curtain and is
collected in the top part of the connecting pipe where the air intake pipe is installed. This occurs in a
single action because the surfacing distance of the bubble is shorter than the air intake pipe.
Therefore, the flow velocity is collection air efficiency in the experiment on 1.0 m/s (0.35 m/s in the
model) and 2.0 m/s (0.71 m/s in the model) which is very similar to types I and II.
On the other hand, when flow velocity is 3.0 m/s (1.06 m/s in the model), the bubble surfacing
distance is 2.8 m (9.3D) as the model value, and the air collection efficiency of type I (the length of the
air intake pipe is 1.91 m as the model value) decreased more than that of type II (pipe length of 3.0 m
as model value) because some of the bubbles pass by the curtain and reach the surface.
[ type Ⅰ]
[ type Ⅱ]
Q1 :Entrained air intake from manhole to connecting pipe
Q3 :Collected air intake from collector pipe
0.008
Entrained air intake and
3
collected air intake(m /s)
Entrained air intake and
3
collected air intake(m /s)
Q1 :Entrained air intake from manhole to connecting pipe
Q3 :Collected air intake from collector pipe
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
0.000
Ra= 5%
10%
Flow velocity in connecting pipe
: 1.0m/s (0.35m/s in model)
5%
10%
2.0m/s
(0.71m/s)
5%
10%
3.0m/s
(1.06m/s)
Ra= 5%
10%
Flow velocity in connecting pipe
: 1.0m/s (0.35m/s in model)
5%
10%
2.0m/s
(0.71m/s)
5%
10%
3.0m/s
(1.06m/s)
Figure 23 Results from air intake experiments
4.4.3 Examination of remedial measures of collection air efficiency
As shown in Figure 24, observation of the inner air intake pipe suggested that water was obstructing
the exhaust, since the water was collected in the refraction part from the air intake pipe to the exhaust
tube while the exhaust was intermittent.
We therefore considered whether air collection efficiency might be improved by using a water
extractor pipe to exclude water in the refraction part. Type A set up the water extractor pipe of
φ 20mm in the inside diameter of the model in the pipe bottom in the air intake pipe refraction part.
The water extractor pipe exit is oriented in the same direction as the connecting pipe downstream.
Type B uses a water extractor pipe in the bottom of the refraction part as does type A, but with the
water draining outside the system. Type B was used for verification when the backflow did not
influence it because in Type A, there was the potential for water to flow backward from the water
extractor pipe (see Figure 25).
9
SESSION 2.3
The experimental results for type II satisfied the bubble surfacing distance at a flow velocity of 3.0
m/s. Figure 26 shows the outcome of the experiment when remedial measures were applied.
The rate has greatly improved to air collection efficiency Q3/Q1 of type II A and II B by 90% or more
in the case with large volumes of air taken from the manhole to the connecting pipe. Moreover, air
collection efficiency is similar for types II A and II B. This was attributed to the effect of the water
extractor pipe and of draining the water outside the system.
Exhaust pipe
Water accumulates at bend
Collector pipe
Air flows downstream
Water surface fluctuates in connecting
pipe and water enters collector pipe
Figure 24 Exhaust obstruction in the intake pipe refraction part
[type A]
[type B]
Collector pipe
Collector pipe
φ31
集気管
φ20
42
φ31
集気管
水抜き管
72
Drain pipe
Expelled from system
28
系外へ排水
Figure 25 Study of modifications to collector pipe (detail of bend in pipe)
[ typeⅡA]
[ typeⅡB]
Q1 :Entrained air intake from manhole to connecting pipe
Q3 :Collected air intake from collector pipe
0.008
Entrained air intake and
collected air intake(m3/s)
Entrained air intake and
collected air intake(m 3/s)
Q1 :Entrained air intake from manhole to connecting pipe
Q3 :Collected air intake from collector pipe
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
0.000
Ra= 5%
10%
Flow velocity in connecting pipe
: 1.0m/s (0.35m/s in model)
5%
10%
2.0m/s
(0.71m/s)
5%
10%
3.0m/s
(1.06m/s)
Ra= 5%
10%
Flow velocity in connecting pipe
: 1.0m/s (0.35m/s in model)
5%
10%
2.0m/s
(0.71m/s)
5%
10%
3.0m/s
(1.06m/s)
Figure 26 Comparison of air collection
4.5
Summary
Through this hydraulics model test, we obtained new findings regarding the extension of pipe length
based on the bubble surfacing distance and the effect of a water extractor pipe in the refraction part of
the system.
However, the results in this study were obtained under a set of given preconditions. We have
therefore not established an idealized geometry. Further research is required into material
specifications and installation methods.
LIST OF REFERENCES
JIWET (Japan Institute of Wastewater Engineering Technology). (1999). Design material concerning
Helicoidal-Ramp Type Drop Shaft. (In Japanese)
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